Description of problem:
Method java.sql.Clob.getSubstring(int,int) returns either empty string (if second argument, length, is 0) or whole string (if length >= 1).

Steps to Reproduce:
> DDL
CREATE FOREIGN TABLE smalla(
    intkey integer PRIMARY KEY OPTIONS(NAMEINSOURCE 'intkey', NATIVE_TYPE 'int4'),
    objectvalue clob OPTIONS(NAMEINSOURCE 'objectvalue', NATIVE_TYPE 'text'))
  OPTIONS(NAMEINSOURCE 'smalla');

> Query:
SELECT * FROM smalla

> Java code:
Connection con = org.teiid.jdbc.TeiidDriver.getInstance().connect("jdbc:teiid:test@mm://localhost:31000", info);
ResultSet rs = con.createStatement().executeQuery("select * from source_pg.smalla");
rs.next();
LOG.info("CLOB: {}", rs.getClob(2).getClass());
LOG.info("CLOB: {}", rs.getClob(2).getSubString(1, 1));
LOG.info("CLOB: {}", rs.getClob(2).getSubString(1, 0));
LOG.info("CLOB: {}", rs.getClob(2).getSubString(1, -1));
con.close();

> Output:
08:03:44.089 [main] INFO  sandbox.MainClass - CLOB: class org.teiid.core.types.ClobImpl
08:03:44.091 [main] INFO  sandbox.MainClass - CLOB: -24
08:03:44.092 [main] INFO  sandbox.MainClass - CLOB: 
Exception in thread "main" java.sql.SQLException: Invalid length argument to getSubString(): -1
	at org.teiid.core.types.ClobImpl.getSubString(ClobImpl.java:154)
	at sandbox.MainClass.main(MainClass.java:81)